50 HP Motor Cuts Off After 30-40 Minutes But Idles Fine

I have a 50 HP motor. After I run it down the lake and let it sit for 30-40 minutes, it cranks ,and idles fine, but when I try to throttle it the motor cuts-off immediately, or I have to play with the throttle. I've had 2 different Marine repair shops look at it. They say they can't duplicate the problem in their tank, so they adjust the carburetors, and change the plugs, but this does not fix the problem. Do you have any suggestions?

[robert graham]

Check your fuel line from the tank, fittings, primer bulb, quick-disconnect on motor, fuel filter, fuel pump. There are several check valves, especially in the primer bulb that must be clean and working for your fuel delivery system to operate and "hold prime" during operation. Good Luck!

[99yam40]

You can test the fuel system to see if that is the problem and if it is not start looking for spark or other problems.
look at this from the frequently ask questions.

HOW TO: Testing your boats fuel system. - iboats Boating Forums